FIG. 17 shows an example of a projection display apparatus using three projection lenses for projecting images of different primary colors, namely images of red, green and blue, onto a screen. The display apparatus includes a screen 101, projection lenses 102-104, projection tubes 105-107, deflection coils 108-110, convergence coils 111-113, deflection circuit 114, and convergence coil driving circuits 115-117 (B-CY, G-CY and R-CY drive circuits).
In the display apparatus, the same deflection waveform is supplied from the deflection circuit 114 to the deflection coils 108-110 which have the same characteristics or standard and which are mounted on the neck portions of the projection tubes 105-107, respectively. As a result, images of different primary colors, namely red, green and blue, are formed on the front surfaces of the projection tubes 105-107, then enlarged by the projection lenses 102-104 and projected on the screen 101 in an overlapped manner, whereby an enlarged color image is formed on the screen.
In such a projection display apparatus, there generally is adopted an in-line arrangement (arrangement in the horizontal direction) of the projection tubes 105-107. Particularly, the green projection tube 106 is disposed at the center, and on opposite sides thereof are disposed respectively the red and blue projection tubes 107, 105. The green projection tube 106 is disposed in a straight manner just in front of the screen, while the red and blue projection tubes 107, 105 are disposed in a slightly inwardly inclined state, that is, inclined toward the green projection tube 106. The angle of this inward inclination is designated as the "convergence angle".
According to the recent type of projection display apparatus, lenses of a short focal length are used as the projection lenses 102-104 to shorten or reduce the projection distance L between those projection lenses and the screen 101 and thereby effect a thinning or depth reduction of the apparatus. However, with shortening of the distance L, not only the projection lenses 102-104 increase in field angle, but also the convergence angle tends to become larger. Consequently, the red and blue images projected on the screen 101 are largely distorted. As a result, the red, green and blue images are no longer registered on the screen.
For correcting such a distortion and registering the red, green and blue images, there has been proposed a construction wherein convergence coils 111-113 are mounted on the neck portions of the projection tubes 105-107, and correction waveforms are fed thereto from convergence coil driving circuit 115-117, respectively, as described in Japanese Patent Publication No. 49433/88. However, when the aforementioned distortion is large, it is required to increase the amount of correction in the convergence coil driving circuits 115-117.
Such an increase of the amount of correction in the convergence coil driving circuits give rise to various problems as follows:
(1) Increase in output of the convergence coil driving circuits 115-117 causes an increase of the power consumption and a great increase of cost. PA1 (2) Increase in magnetic field of the convergence coils 111-113 causes an increase of electron beam spot distortion and deterioration of resolution. PA1 (3) Increase in the amount of correction of the convergence coil driving circuits 115-117 gives rise to a change in scanning lines of electron beam, thus causing deterioration of S/N ratio. PA1 (4) Increase in the amount of heat generated in the convergence coils 111-113. PA1 (a) a method wherein the red and blue projection tubes 107, 105 are disposed without inclination to prevent distortion of the projected image on the screen 101, as described in Japanese Patent Publication No. 47518/84; and PA1 (b) a method wherein a portion of the distortion based on the convergence angle is corrected beforehand in the deflection circuit, as described in Japanese Patent Application Laid-Open No. 48872/82.
Heretofore, for solving the above-mentioned problems, there have been adopted, for example:
According to the above method (a), when the projection distance is extremely short, the raster size on each projection tube becomes extremely small, so that there arises another problem relating to brightness and resolution. And according to the above method (b), it is impossible to obtain a satisfactory effect of correction.
Red and blue raster distortions induced by the increase of field angle or convergence angle can be broadly classified into two distortion components--keystone distortion and horizontal linearity distortion. Particularly, the increase of the horizontal linearity distortion is conspicuous.
Another technique is described in Japanese Patent Application Laid-Open No. 56590/83. According to this technique, in order to solve the above-mentioned problems, a linearity correction arrangement is connected to each deflection coil mounted on projection tubes.
FIG. 18 is a circuit diagram showing an example of three linearity correction coils used in a projection display apparatus having a horizontal drive pulse input terminal 1, a horizontal output transistor 2, a damper diode 3, a resonance capacitor 4, a power supply terminal 5, a transformer 6, horizontal deflection coils 7-9, linearity correction coils 10-12, and an S-correction capacitor 13. The horizontal output transistor 2, damper diode 3 and resonance capacitor 4 constitute a portion of a horizontal deflection circuit, and a saw tooth-like horizontal deflection current is formed by switching the horizontal output transistor 2 in accordance with a horizontal drive pulse of a horizontal scan period provided from the input terminal 1.
The horizontal deflection coil 8 is mounted on the neck portion of a projection tube corresponding to the green projection tube 106 shown in FIG. 17, which projection tube associated with the coil 8 will hereinafter be referred to as the projection tube 106, correspondingly to FIG. 17, for ease of explanation. In series with the horizontal deflection coil 8 is connected to the linearity correction coil 11. Likewise, the horizontal deflection coils 7 and 9 are mounted on the neck portions of projection tubes corresponding to the red and blue projection tubes 107, 105, respectively, shown in FIG. 17, which projection tubes associated with the coils 7 and 9 will hereinafter be referred to as the projection tubes 107 and 105, correspondingly to FIG. 17, for ease of explanation. In series with the horizontal deflection coils 7 and 9 are connected the linearity correction coils 10 and 12, respectively.
The horizontal deflection coils 7 to 9 are connected to the power supply terminal 5 through the transformer 6, while the linearity corrections coils 10 to 12 are grounded through the S- correction capacitor 13. The foregoing horizontal deflection current is fed to the horizontal deflection coils 7-9 simultaneously. The linearity corrections coils 10, 11 and 12 serve for correcting the horizontal linearity of the projection tubes 107, 106 and 105, respectively. For example, they are magnetically biased and possess the following characteristics.
The linearity correction coil 11 connected to the horizontal deflection coil S has a characteristic indicated by curve 22 in FIG. 19. FIG. 19 represents the magnitude of inductance of each linearity correction coil relative to the magnitude of DC superimposed current in the coil. In the case where the linearity correction coil 11 is disposed as in FIG. 18, the DC superimposed current corresponds to the foregoing horizontal deflection current, which is at a maximum negative level at the beginning of the horizontal scan and shifts from zero to the positive side as the horizontal scan proceeds. Thus, when the DC superimposed current is negative, the horizontal scan is at its first half period, and the DC superimposed current becomes positive at the second half of the horizontal scan period.
As is apparent from the characteristic curve 22 shown in FIG. 19, the linearity correction coil 11 exhibits a large inductance at the first half of the horizontal scan period so that the effect of linearity correction is greater than at the second half of the same period. The reason why the effect of linearity correction is made different between the first and second halves of the horizontal scan period is that at the latter half of the same period, the saturation voltage of the horizontal output transistor 2 becomes high and so it is intended to prevent the horizontal deflection current of saw-tooth shape from assuming a waveform which shows a tendency for saturation. In the case where a plurality of vertical lines to be displayed as lines spaced at equal intervals are displayed as an image 25 on the front surface of the projection tube 106, as shown in FIG. 20, the vertical line intervals are almost constant as a result of the linearity correction made above.
The linearity correction coil 10 connected to the horizontal deflection coil 7 has a characteristic indicated curve 21 in FIG. 19. Thus, at the first half of the horizontal scan period, the inductance of the coil 10 is smaller than that of the coil 11, that is, the linearity correction is less effective. Consequently, as shown in FIG. 20, an image 24 on the front surface of the projection tube 107 extends in the horizontal direction at the first half of the horizontal scan period and contracts in the same direction at the second half of the same period.
The linearity correction coil 12 connected to the horizontal deflection coil 9 has a characteristic indicated by curve 23 in FIG. 19. More particularly, at the first half of the horizontal scan period, the inductance of the coil 12 is larger and the effect of linearity correction is greater in comparison with the linearity correction coil 10. Consequently, an image 26 on the front surface of the projection tube 105 contracts in the horizontal direction at the first half of the horizontal scan period and extends in the same direction at the second half of the same period, as shown in FIG. 20.
The image 25 on the projection tube 106, shown in FIG. 20, is projected enlargedly onto a screen by a projection lens to afford such a raster 28, as shown in FIG. 21. The image 24 on the projection tube 107, shown in FIG. 20, is also projected enlargedly onto the screen by a projection lens to afford such a raster 29, as shown in FIG. 21. Further, the image 26 on the projection tube 105, shown in FIG. 20, is also projected enlargedly onto the screen of a projection lens to afford such a raster, as shown in FIG. 21. The horizontal linearity of the raster 29 and that of the raster 27 can be made approximately coincident with that of the raster 28 although keystone distortion remains therein.
Each linearity correction arrangement, as indicated above, acts on a deflecting coil corresponding to a projection lens arrangement, wherein the raster extends at the first half of the electron beam scan period, in such a manner that the inductance thereof is large at the first half of the electron beam scan period, and becomes small at the second half of the same period, to correct the linearity distortion of the projection optical system. 0n the other hand, it acts on a deflecting coil corresponding to a projection lens arrangement, wherein the raster extends at the latter half of the electron beam scan period, in such a manner that the inductance thereof is small at the first half of the same period to correct the linearity distortion of the projection optical system.
Therefore, it is no longer necessary to make correction of the horizontal linearity distortion with respect to the correction signal in each convergence coil driving circuit, whereby the burden on the convergence coil driving circuit can be diminished and, hence, the power consumption can be reduced, and at the same time, it is made possible to improve the resolution and S/N ratio.